Merge pull request #3208 from MicrosoftDocs/master

10/14/2020 AM Publish

Author: v-alje

docs/c-language/c-keywords.md

@@ -1,7 +1,7 @@
 ---
 title: "C Keywords"
 description: "Keywords in Standard C and Microsoft C compiler extensions."
-ms.date: 09/12/2020
+ms.date: 10/12/2020
 helpviewer_keywords: ["keywords [C]", "redefining keywords", "Microsoft-specific keywords"]
 ms.assetid: 2d932335-97bf-45cd-b367-4ae00db0ff42
 ---
@@ -110,7 +110,8 @@ The following keywords and special identifiers are recognized by the Microsoft C
         **`__try`**<sup>5</sup>\
         **`dllexport`**<sup>4</sup>\
         **`__inline`**<sup>5</sup>\
-        **`__leave`**<sup>5</sup>
+        **`__leave`**<sup>5</sup>\
+        **`static_assert`**<sup>6</sup>
     :::column-end:::
 :::row-end:::
 
@@ -120,6 +121,8 @@ The following keywords and special identifiers are recognized by the Microsoft C
 
 <sup>5</sup> For compatibility with previous versions, these keywords are available both with two leading underscores and a single leading underscore when Microsoft extensions are enabled.
 
+<sup>6</sup> When <assert.h> is not included, the Microsoft Visual C compiler maps **`static_assert`** to the C11 **`_Static_assert`** keyword.
+
 Microsoft extensions are enabled by default. To assist in creating portable code, you can disable Microsoft extensions by specifying the [/Za \(Disable language extensions)](../build/reference/za-ze-disable-language-extensions.md) option during compilation. When you use this option, some Microsoft-specific keywords are disabled.
 
 When Microsoft extensions are enabled, you can use the keywords listed above in your programs. For standards compliance, most of these keywords are prefaced by a double underscore. The four exceptions, **`dllexport`**, **`dllimport`**, **`naked`**, and **`thread`**, are used only with **`__declspec`** and don't require a leading double underscore. For backward compatibility, single-underscore versions of the rest of the keywords are supported.

docs/c-language/static-assert-c.md

@@ -0,0 +1,79 @@
+---
+title: "_Static_assert keyword and static_assert macro (C11)"
+description: "Describes the C11 _Static_assert keyword and the C11 static_assert macro."
+ms.date: "10/13/2020"
+f1_keywords: ["static_assert_c", "_Static_assert"]
+helpviewer_keywords: ["assertions [C], _Static_assert, static_assert"]
+---
+
+# _Static_assert keyword and static_assert macro (C11)
+
+Tests an assertion at compile time. If the specified constant expression is **`false`**, the compiler displays the specified message and the compilation fails with error C2338; otherwise, there's no effect. New in C11.
+
+**`_Static_assert`** is a keyword introduced in C11.
+**`static_assert`** is a macro, introduced in C11, that maps to the **`_Static_assert`** keyword.
+
+## Syntax
+
+```C
+_Static_assert(constant-expression, string-literal);
+static_assert(constant-expression, string-literal);
+```
+
+### Parameters
+
+*constant-expression*\
+An integral constant expression that can be evaluated at compile time. If the expression is zero (false), displays the *string-literal* parameter and the compilation fails with an error. If the expression is nonzero (true), then there's no effect.
+
+*string-literal*\
+The message displayed if *constant-expression* evaluates to zero (false). The message must be made using the [base character set](../c-language/ascii-character-set.md) of the compiler. The characters can't be [multibyte or wide characters](../c-language/multibyte-and-wide-characters.md).
+
+## Remarks
+
+The **`_Static_assert`** keyword, and the **`static_assert`** macro, both test a software assertion at compile time. They can be used at global or function scope.
+
+In contrast, the [assert macro and _assert and _wassert functions](../c-runtime-library/reference/assert-macro-assert-wassert.md) test a software assertion at runtime and incur a runtime cost.
+
+**Microsoft-specific behavior**
+
+In C, when you don't include `<assert.h>`, the Microsoft Visual C/C++ compiler treats **`static_assert`** as a keyword that maps to **`_Static_assert`**. Using **`static_assert`** is preferred because the same code will work in both C and C++.
+
+## Example of a compile-time assert
+
+In the following example, **`static_assert`** and **`_Static_assert`** are used to verify how many elements are in an enum and that integers are 32 bits wide.
+
+```C
+// requires /std:c11 or higher
+#include <assert.h>
+
+enum Items
+{
+    A,
+    B,
+    C,
+    LENGTH
+};
+
+int main()
+{
+    // _Static_assert is a C11 keyword
+    _Static_assert(LENGTH == 3, "Expected Items enum to have three elements");
+
+    // Preferred: static_assert maps to _Static_Assert and is compatible with C++
+    static_assert(sizeof(int) == 4, "Expecting 32 bit integers"); 
+
+    return 0;
+}
+```
+
+## Requirements
+
+|Macro|Required header|
+|-------------|---------------------|
+|**`static_assert`**|\<assert.h>|
+
+## See also
+
+[_STATIC_ASSERT Macro](../c-runtime-library/reference/static-assert-macro.md)\
+[assert macro and _assert and _wassert functions](../c-runtime-library/reference/assert-macro-assert-wassert.md)
+[/std (Specify language standard version)](../build/reference/std-specify-language-standard-version.md)

docs/c-language/toc.yml

@@ -400,6 +400,8 @@
         href: ../c-language/null-statement-c.md
       - name: return statement (C)
         href: ../c-language/return-statement-c.md
+      - name: static_assert statement (C11)
+        href: ../c-language/static-assert-c.md
       - name: switch statement (C)
         href: ../c-language/switch-statement-c.md
       - name: try-except statement (C)